What is the difference between proportional control and on/off control?

An on/off valve opens or closes fully; in proportional control joystick travel becomes speed and the load is positioned precisely, without being jolted.

An on/off control valve either opens fully or closes fully; the movement starts at full speed and stops abruptly when you release it. In proportional control, how far you push the joystick determines the speed of the movement: push it a little and it goes slowly, push it a lot and it goes fast. In one sentence the difference is this: on/off answers the question "should it go", proportional answers the question "how fast should it go".

The two forms of control compared

The difference shows most at the moment of stopping. In on/off control, releasing the movement means closing the valve all at once. The moving load stops, but the speed it carried does not go anywhere; the boom, the bucket or the suspended load swings and the operator waits for that swing to die down. In proportional control, when the joystick is brought slowly back to centre the flow decreases gradually, and by the time the load stops it has no energy left to swing with. In loader, bucket and crane applications, even when the lifting speed is the same, the total working time differs because of the wait for the swing.

How proportional control is obtained

There are two ways. The first is to use a proportional valve on the hydraulic side: the coil current determines the spool opening, and that in turn determines the flow. The relationship between them is explained in the difference in flow characteristic between a proportional and an on/off valve. The second is to drive the existing mechanical lever proportionally: as long as the servo attached to the lever can hold it in intermediate positions, the system behaves proportionally. In both cases what produces the command is the proportional joystick; proportional control cannot be obtained with an on/off button. Nor does the characteristic have to be linear; a parabolic joystick characteristic gives finer adjustment in the region near the centre.

When on/off is enough

Not every job asks for proportional control. In applications where the movement is either fully open or fully closed, where the stroke is short and where the load swinging causes no problem, on/off is both sufficient and simpler. A hatch working on/off, a support leg that is fixed in place, or a movement whose position is locked with a detent fall into this group; a valve detent does not expect a continuous command in any case. The price paid for proportional control is complexity: a parameter to be set, a joystick to be calibrated and a driver layer to be troubleshot come with it. If the return is not going to be taken, that price is unnecessary.

What proportional control does not solve on its own

Setting up a proportional system does not mean that every joystick movement will turn directly into movement. Because of spool overlap, nothing may happen over the first part of the joystick travel; this is called deadband and it has to be compensated for. If the dead zone at the centre of the joystick is chosen too narrow, movement starts by itself under vibration; that is what the centre dead zone setting is there for. If smoothness is wanted, the ramp time is set as well; the ramp is not an automatic by-product of proportional control.

What to do

Make the decision by looking at the job itself: do you need to stop the load at a particular point, or is it enough for it to travel between the end points? If the stopping point matters, proportional control returns what it costs. If the end points are enough, stay with the on/off solution; its installation, its adjustment and its fault tracing are all easier.

The full topic: Converting a tractor hydraulic lever to electronic control

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